1. Field of the Invention
The present invention relates to a power supply circuit used in various kinds of electronic apparatuses. More particularly, it relates to a circuit for controlling a direct-current to direct-current conversion so as to keep constant a voltage or voltages used in an electronic apparatus, and to a direct-current to direct-current conversion apparatus using the same. Note that, in the description below, "direct-current to direct-current conversion" is simply abbreviated as ""DC/DC"".
2. Description of the Related Art
A portable electronic apparatus such as a so-called hand-held type personal computer is equipped with a battery as its power source. Since the voltage of a battery is generally lowered as the battery discharges, a DC/DC apparatus is incorporated into the electronic apparatus so as to keep constant the output voltage of the battery.
On the other hand, the power source used in an electronic apparatus usually requires a plurality of power supplies, not a single power supply. To this end, a plurality of DC/DC apparatuses are provided for the plurality of power supplies. In this case, if turn-on/turn-off sequences between the respective power supplies are not fully taken into consideration, a drawback occurs in that a latch-up phenomenon is caused in semiconductor devices used in the electronic apparatus and thus some devices are burned out. Accordingly, some ideas are required for controlling power turn-on/turn-off sequences.
In one example of the prior art, the DC/DC apparatus is provided with a special logic circuit for controlling such power turn-on/turn-off sequences. However, this leads to a problem in that the scale of the entire circuit becomes large and the circuit constitution also becomes relatively complicated.
Also, where respective power supplies are simultaneously turned on with respect to a plurality of DC/DC apparatuses, it is substantially impossible to control respective rise characteristics of output voltages of the DC/DC apparatuses since the rise characteristics depend on lightness or heaviness of respective loads of the DC/DC apparatuses. A similar problem also occurs in the case where respective power supplies are simultaneously turned off.
The problems encountered in the prior art will be explained later in detail in contrast with preferred embodiments of the present invention.